Natural gas (“NG”) is routinely transported from one location to another location in its liquid state as “Liquefied Natural Gas” (LNG). Liquefaction of the natural gas makes it more economical to transport as LNG occupies only about 1/600th of the volume that the same amount of natural gas does in its gaseous state. After liquefaction, LNG is typically stored in cryogenic containers either at or slightly above atmospheric pressure. LNG is regasified before distribution to end users through a pipeline or other distribution network at a temperature and pressure that meets the delivery requirements of the end users.
Wellhead gas is subjected to gas pre-treatment to remove contaminants prior to liquefaction. The hydrogen sulphide and carbon dioxide can be removed using a suitable process such as amine adsorption. Removal of water can be achieved using conventional methods, for example, a molecular sieve. Depending on the composition of contaminants present in the inlet gas stream, the inlet gas stream may be subjected to further pre-treatment to remove other contaminants, such as mercury and heavy hydrocarbons prior to liquefaction. Liquefaction is achieved using methods that are well established in the art which typically involve compression and cooling. Such processes include the APCI C3/MR™ or Split MR™ or AP-X™, processes, the Phillips Optimized Cascade Process, the Linde Mixed Fluid Cascade process or the Shell Double mixed Refrigerant or Parallel Mixed Refrigerant process. Regardless of the choice of liquefaction process, refrigerants are used to reduce the temperature of the treated wellhead gas to a temperature of around −160° C. to form LNG, resulting in warming of the refrigerant which must be compressed for recycle to the liquefaction process. The compressors used for this duty are traditionally gas turbines or electric motors depending on the power requirements and layout issues of a particular LNG production facility. The coolers required for the various compression and heat exchanger operations associated with an LNG plant may be air coolers or water coolers arranged in a heat exchanger bank.
Prior art modularised LNG production trains have been closely based upon the design and layout of the more traditional stick-built LNG production trains. Until now, modularisation has been conducted by slicing up an existing stick built LNG train design into transportable sections, leading to some compromises regarding the placement of the module boundaries. Prior art examples of modularization of a traditional stick-built air-cooled LNG train have relied on dividing the air-cooled heat exchanger bank into the smallest number of modules possible for a given size of air cooler within the air-cooled heat exchanger bank. To keep the overall plot size of the LNG production facility to a minimum, it is known to arrange sub-sections of the air-cooled heat exchanger bank over the top of each module so as to cover one hundred percent of the area defined by the base of said module with a view to making the air-cooled heat exchanger bank as large as possible for a given module size. Having made the decision to fully cover each of the modules with a portion of the air-cooled heat exchanger bank, selected larger or taller pieces of process equipment operatively associated with each module, such as pressure vessels, compressors and the cryogenic heat exchanger are either stick built or constructed as separate modules which are designed to remain uncovered by the air-cooled heat exchanger bank.
The overall footprint of such modularised LNG production plants is large because sufficient plot space needs to be allocated to allow for covered modules incorporating the air-cooled heat exchanger bank to be positioned in a straight line running along the central longitudinal axis of the LNG production facility with the uncovered modules being offset from the central longitudinal axis and located on one side or the other side of the centrally located air-cooled heat exchanger bank. This prior art arrangement has several disadvantages. A high number of interconnections are required across the modules between the air-cooled heat exchanger bank covered modules and the associated equipment located on an adjacent uncovered module. The use of a large number of small modules inevitably requires that the air coolers within the air-cooled heat exchanger bank that are required to perform cooling duty for a particular module will need to span across at least two modules, preventing fluid circulation through the air coolers until these two modules are joined at the production location. These prior art designs rely on duplication of structural steel as there is inevitably a large amount of void space underneath the air-cooled heat exchanger bank in addition to the structural steel that is used for the uncovered spatially offset process equipment modules.
There remains a need to explore alternative designs for a modular LNG production plant to alleviate at least one of these problems.